Testing the cation-hydration effect on the crystallization of Ca-Mg-CO3 systems

Area of Science:

• Geochemistry and Mineralogy
• Materials Science
• Biomineralization Chemistry

Background:

• Dolomite and magnesite are common Earth surface minerals, but their synthesis at ambient conditions has been challenging.
• Previous research attributed synthesis difficulties to the strong solvation shells of magnesium ions in aqueous solutions.

Purpose of the Study:

• To investigate the synthesis of magnesium carbonate (MgCO3) and mixed calcium-magnesium carbonate (MgxCa(1-x)CO3) phases.
• To explore mineral formation in an anhydrous environment, challenging the role of cation hydration.

Main Methods:

• Experiments were conducted in a dry organic solvent, excluding water.
• Varying molar percentages of Mg(2+) and Ca(2+) ions were used in the solutions.
• Characterization of precipitated solid phases, including crystallinity and composition.

Main Results:

• Anhydrous MgCO3 and MgxCa(1-x)CO3 phases readily precipitated in the water-free environment.
• Crystallinity of precipitates was highly dependent on the Mg molar percentage.
• Low [Mg2+]/[Ca2+] ratios favored magnesian calcite, while high ratios and pure Mg solutions yielded amorphous phases or MgCO3. At a 1:1 ratio, both protodolomite nanocrystals and amorphous phases formed.

Conclusions:

• An intrinsic barrier exists for Mg2+ and CO3(2-) to form ordered structures at ambient conditions, independent of hydration.
• The long-held belief that cation hydration solely inhibits dolomite and magnesite formation needs reevaluation.
• Findings offer insights into the 'dolomite problem,' biomineralization, and mineral carbonation.